This invention relates generally to electrostatic chucks used to secure semiconductor wafers in processing chambers and, more particularly, to significant improvements that enhance wafer cooling and increase the useful life of chucks of this type.
Electrostatic chucks are used in semiconductor processing chambers to hold individual semiconductor substrates or wafers on a chamber pedestal. An electrostatic chuck typically includes a dielectric layer and an electrode. A semiconductor wafer is placed in contact with the dielectric layer and a de voltage is applied to the electrode to create an electrostatic attractive force to grip the wafer to the chuck. Chucks of this type are particularly useful in vacuum processing environments in which there is insufficient differential pressure to hold the wafer in position, or in which mechanical clamping of the wafer is undesirable. One processing environment in which the electrostatic chuck is widely used is the plasma etch process.
Although an electrostatic chuck may be formed using only a single dielectric layer and an electrode, a more typical configuration includes a thin laminate member having an electrode core, which is preferably a thin copper layer, sandwiched between upper and lower dielectric layers or an organic material such as polyimide. A polyimide adhesive may be used to assemble these layers into a single laminate sheet, and to attach the laminate the chamber pedestal.
The upper and lower polyimide layers merge at the circumferential edge of the electrode to prevent exposure of the copper electrode to the process chamber, which usually contains a plasma that would damage the copper. In operation, the copper electrode is connected to a voltage source and functions as an anode. The principles of operation of electrostatic chucks are well known and not critical to the present invention. It is sufficient to note that an electric field formed above the pedestal results in mutual attraction between the pedestal and a semiconductor wafer placed in contact with the laminate.
Many processes to which wafers are subjected result in the generation of heat, and means must be provided to cool the wafer to an acceptable process temperature to prevent heat damage. In most electrostatic chucks of the prior art, helium gas is introduced beneath the wafer through a central aperture in the pedestal, and is then distributed through a pattern of grooves in the laminate on the pedestal. Various groove patterns have been used in an effort to distribute the cooling gas uniformly across the wafer surface, which is generally circular in shape. A difficulty inherent to this cooling technique prevents uniform cooling near the wafer edge. If the grooves are continued all the way to wafer edge, there is a high rate of gas leakage and an accompanying reduction in the cooling effect, especially near the edge. In most designs, the grooves are terminated before they reach the edge. In some designs, radial grooves adjoin secondary grooves near the edge, and the secondary grooves may extend circumferentially for short distances before terminating, or may take the form of branch grooves extending in various directions before terminating. In all these designs, however, there is still undesirable overheating of the wafer near its peripheral edge.
Another drawback of electrostatic chucks of the prior art arises from the way the laminate of polyimide and copper is formed on the chuck pedestal. A first layer of polyimide is placed over the pedestal and then the copper electrode is placed over the polyimide. Of course, the copper layer cannot extend all the way to the edge of the polyimide, because the polyimide layers have to overlap and provide a seal around the outer edge of the copper. When the second polyimide layer is placed over the copper, the total thickness of the laminate is smaller around the polyimide edge (the thickness of two polyimide layers) than the total laminate thickness including the copper. Therefore, the laminate has an annular step outside the outer diameter of the copper layer. This annular step has two detrimental effects on the performance of the electrostatic chuck. First, because of the step there is only a very small region of the chuck providing contact with the wafer beyond the outer diameter of the copper layer. As a result, there is a high probability of helium leakage at the wafer edge, and unwanted overheating of the wafer edge region. A second effect of the annular step in the laminate structure is that the outer edge of the copper layer is insulated from the damaging chamber environment by only a very thin polyimide layer, having a thickness approximately the same as that of the upper polyimide layer. Once this layer is eroded away by the process plasma in the chamber, the chuck has to be replaced. Polyimide and other organic materials have a relatively low tolerance for many process gases and plasmas. Therefore, provision of a good insulating layer around the electrode is an important consideration.
It will be appreciated from the foregoing that there is a need for improvement in electrostatic chucks. In particular what is needed is an improved chuck construction to enhance cooling near the peripheral edge of the wafer and to lengthen the useful life of the chuck. The present invention satisfies this need, as will be apparent from the following summary.